That's not in any way relevant at all, or even similar. This isn't about another car - it would be the same car. It's more like you advertising your car for £3,000 and I advertise the exact same car for £3,000 - and then when someone comes to me, I give them a lift to your house and charge them £50 for the ride.
Trainline just sells the same tickets everyone else does. If there's a ticket that's 50% cheaper than a walk-on fare, there's a ticket that's 50% cheaper and you can get it from, presumably, anyone that sells rail tickets. That's the whole point - they haven't negotiated special deals or tickets you can't get anywhere else (such as an exclusive allocation of cheap tickets on certain train services). If they did, they'd actually have an edge.
Even though you quoted the bit where I said the ASA couldn't do anything, you then asked why I thought they could!! Did you read it all, or just click reply as soon as you read about five words?!
I don't care if Trainline wants to spend a fortune advertising to get people to buy their tickets from them (and many TOCs do indeed use them too) and claim to find you the best ticket, but the advertising IMPLIES that you'll only save money by going to them. That isn't true, and the wording doesn't actually say that they are exclusive in any way either, but it's pretty obvious to anyone that it is what they're trying to make people think.
In any case, once you add on the booking fee, they'll always be more expensive than some other places. And since when did it become impossible to get advanced tickets from a train station, in advance? You can't claim that the only way you can get advanced tickets is to go to Trainline?